In some cases, interferometric measuring systems monitor changes in the relative position of a measurement object based on an optical interference signal. For example, an interferometer generates the optical interference signal by overlapping and interfering a “measurement beam” that interacts with (e.g., reflects from) the measurement object with a second beam, sometimes called a “reference beam.” Changes in the relative position of the measurement object correspond to changes in the phase of the measured optical interference signal.
However, many interferometric measuring systems include nonlinearities such as what are known as “cyclic errors.” In general, cyclic errors are understood as measurement errors that are periodic with the relative position of a measurement object. The cyclic errors can be expressed as contributions to the phase and/or the intensity of the measured interference signal and have a sinusoidal dependence on the change in an optical path difference between the measurement beam and the second beam. The cyclic errors can be produced by “beam mixing,” in which a portion of an input beam that nominally forms the second beam propagates along a measurement path and/or a portion of an input beam intended to propagate along the measurement path instead propagates along a reference path. Such beam mixing can be caused by ellipticity in the polarizations of the input beam and/or imperfections in the interferometer components. Cyclic errors can also be produced by imperfections in components such as retro-reflectors and/or phase retardation plates that produce undesired ellipticities in beams in the interferometer. If not compensated, the foregoing cyclic errors can limit the accuracy of position changes measured by the interferometer system.